console: add API to get underlying gui window ID

This adds two console functions, qemu_console_set_window_id and
qemu_graphic_console_get_window_id, to let graphical backend record the
window id in the QemuConsole structure, and let the baum driver read it.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Message-id: 20161221003806.22412-2-samuel.thibault@ens-lyon.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>